Method and an apparatus for compression moulding an object made of polymeric material

ABSTRACT

A method for producing an object made of a polymeric material, said polymeric material having a melting temperature (T F ), comprises the steps of: melting the polymeric material; after the step of melting, cooling the polymeric material below the melting temperature (T F ) in a cooling zone; severing a dose from a flow of polymeric material coming from the cooling zone by means of a severing element ( 5 ); obtaining said object by shaping the dose between a male forming element and a female forming element ( 10 ) which move towards one another with a mutual movement speed, the dose having a temperature lower than said melting temperature (T F ). Between a start-of-forming configuration, in which the dose is in contact with both the male forming element and the female forming element ( 10 ), and a start-of-deceleration configuration, in which the male forming element and the female forming element ( 10 ) begin to decelerate relative to one another, the mutual movement speed is greater than 10 mm/s.

The invention relates to a method and an apparatus for producing objectsby compression moulding a polymeric material. The objects which can beproduced by means of the method and the apparatus according to theinvention may comprise, by way of example, caps for containers, preformsfor obtaining containers by blow moulding or stretch-blow moulding, orcontainers. The polymeric material which may be processed by the methodand the apparatus according to the invention can be any materialsuitable for use in compression moulding, in particular asemi-crystalline material such as polypropylene (PP), high densitypolyethylene (HDPE) or polyethylene terephthalate (PET). More broadlyspeaking, the method and the apparatus according to the invention can beused for processing any polymeric material having a melting temperaturehigher than its crystallization temperature and/or glass transitiontemperature.

Traditionally, the objects obtained by compression mouldingsemi-crystalline polymeric materials are produced by inserting into amould a dose of polymeric material, the dose having a temperature higherthan the respective melting temperature. The dose is shaped between amale element and a female element of the mould so as to obtain thedesired object, which is then cooled inside the mould and then extractedtherefrom.

EP 1265736, which belongs to the same patent family as WO 01/66327,discloses a method for producing an object by compression moulding asemi-crystalline polymeric material, wherein the polymeric material isheated inside of an extruder until the polymer material reaches atemperature higher than the melting temperature thereof. Subsequently,the polymeric material is cooled at a working temperature which is lowerthan the melting temperature, but higher than the crystallization starttemperature at which crystallization begins during the cooling process.From the polymeric material so cooled, doses are obtained having apreset mass. The doses are inserted into respective moulds in which theyare shaped between a male element and a female element. While thepolymeric material is being shaped in the mould, temperature thereof ismaintained at a value close to the crystallization start temperature.Subsequently, the object obtained by shaping the polymeric material, iscooled and then removed from the mould.

U.S. Pat. No. 4,874,571 discloses an apparatus for calendering apolymeric web coming out of an extruder, the polymeric web being made ofa semi-crystalline polymeric material. The polymeric web is caused topass between a first calendering roller and a second calendering roller.Thereafter, the polymeric material is wound around the secondcalendering roller which has a temperature of 180° C. or more. In thenip between the first calendering roller and the second calenderingroller, there is provided a blowing device for emitting a cooled gaswhich helps the polymeric web to detach from the first calenderingroller and to adhere to the second calendering roller. The flow rate ofgas coming out of the blowing device is adjusted so as to attain anoptimal transparency of the calendered web.

The blowing device is configured in such a manner that the polymericweb, for example in the case of polypropylene, does not cool below atemperature of 180° C., i.e. the polymeric web remains at a temperaturehigher than its melting temperature, which for polypropylene is about165° C.

WO 87/04387 discloses a method for a solid-state stamping of afiber-reinforced thermoplastic composite material. The compositematerial at issue comprises a matrix consisting of a semi-crystallinethermoplastic material. According to this method, the composite materialis heated in an oven up to a temperature which is lower than its meltingpeak temperature. From the oven, the composite material is thentransferred into a mould, thus allowing the thermoplastic materialconstituting the matrix to cool.

Finally, the composite material is moulded into the mould at atemperature which, at the beginning of the moulding step, is lower thanthe melting peak temperature, but higher than the crystallization starttemperature of the composite.

Documents U.S. Pat. No. 4,874,571 and WO 87/04387 do not provide anyuseful information for improving the known processes aimed at obtainingobjects by compression moulding, since said documents refer to processessuch as calendering and forming of composite materials having nothing incommon with compression moulding of doses of polymeric material.

On the other hand, the method disclosed in EP 1265736 allows the cycletime to be reduced if compared with conventional methods in which thedose is introduced into the mould at a temperature higher than themelting temperature. Indeed, by introducing the dose of polymericmaterial into the mould at a working temperature which is lower than themelting temperature, but slightly higher than the start crystallizationtemperature, a reduction is obtained of the time which is required forcooling the moulded object from the working temperature to a temperatureat which the moulded object can be extracted from the mould withoutbeing damaged. However, the method disclosed in EP 1265736 can befurther improved, particularly with regard to a further reduction of thetime required to compression mould an object, i.e. of the cycle time,but also as regards obtainment of moulded objects exhibiting a ratherhigh crystallinity degree and/or a relatively high molecular orientationand, consequently, good mechanical properties.

WO 2008/118643 discloses a process for producing a polymer article whichis filled and oriented, wherein the polymer is die drawn for inducingpolymer orientation and cavitation.

It is an object of the present invention to improve the methods andapparatuses for producing objects by compression moulding doses ofpolymeric material, particularly of semi-crystalline thermoplasticmaterial. A further object is to speed up the production of objects madeof polymeric material by compression moulding.

Another object is to increase the productivity of apparatuses forproducing objects by compression moulding of doses of polymericmaterial.

Still a further object is to increase the crystallinity and/or themolecular orientations of a compression moulded object.

In a first aspect of the invention, there is provided a method forproducing an object made of a polymeric material, said polymericmaterial having a melting temperature, the method comprising the stepsof:

-   -   melting the polymeric material;    -   after the melting step, cooling the polymeric material below the        melting temperature in a cooling zone;    -   severing a dose from a flow of polymeric material coming from        the cooling zone, by means of a severing element;    -   obtaining said object by shaping the dose between a male forming        element and a female forming element movable relative to one        another, the dose having a temperature lower than said melting        temperature,        wherein the flow of polymeric material moves with an advancement        speed along a path passing through the cooling zone and reaching        the severing element, said advancement speed being calculated as        an average value in a cross-section taken perpendicularly to an        advancement direction of said flow, said advancement speed being        greater than 1.5 mm/s along at least 70%, preferably 90%, of        said path.

Owing to the first aspect of the invention, the productivity may beconsiderably increased compared to the methods of the prior art. Inparticular, the time required for producing an object made of apolymeric material by shaping it between the male forming element andthe female forming element, is drastically reduced if compared with themethods of the known type.

Indeed, by positioning the dose in a mould between the male formingelement and the female forming element at a temperature lower than themelting temperature, a reduction is obtained of the time which isrequired for cooling the formed object until a temperature is reached,at which said object can be handled and then extracted from the mouldwithout being damaged. A reduction of the cycle time is thus obtained.

This time reduction increases, as the working temperature which can beadopted in the mould decreases, i.e. as there is a decrease in thetemperature that the polymeric material can reach while being shapedbetween the male forming element and the female forming element. Inparticular, the temperature that the polymeric material has while beingshaped between the male forming element and the female forming element,is maintained above the crystallization start temperature at whichcrystals begin to form within the polymeric material constituting thedose, in static conditions.

By adopting a high advancement speed upstream of the severing element,the molecular chains which are present in the polymeric material arebrought into a condition of great agitation which makes it moredifficult to freeze these chains in a crystallized condition. A decreasein the crystallization start temperature therefore occurs, so that thepolymeric material may have relatively low temperatures while it isbeing shaped in the mould.

In addition, a quick advancement of the polymeric material causes a FlowInduced Crystallization to occur therein, that is to say, it causesacceleration of the kinetics of crystallization to occur, so that themoulded object can more quickly achieve a semi-crystalline state inwhich it exhibits a sufficient stiffness for being extracted from themould without suffering any damages.

In fact, although the polymeric material is advanced quickly from thecooling zone toward the severing element, start-crystallization nucleiare formed within the flow of polymeric material. However, these nucleiare not capable of completing crystallization due to the highadvancement speed. The high advancement speed ensures that such nucleibecome aligned in an ordered manner, so that they can crystallizerapidly during moulding.

In other words, an increase in crystallinity and/or in molecularorientation of the moulded object occurs.

It was surprisingly found that a 10% reduction of the temperature thatthe dose has when it is introduced into the mould, leads to a 100%increase in productivity with respect to the prior art.

Further, in average, lower temperatures can also be maintained in thecooling zone and up to the severing element since, as already described,by increasing the advancement speed of the flow of polymeric material,the crystallization start temperature is decreased. This allows thepolymeric material to be handled more easily, since it exhibits a higherviscosity and therefore a lower tackiness. It follows that the severingelement can be simplified as well as the devices carrying the dosetowards the mould and inserting the dose inside the mould.

It is also possible to work closer to the crystallization temperature,without incurring in a premature crystallization of the polymericmaterial.

In one embodiment, there is provided the step of accelerating the flowof polymeric material prior to severing the dose therefrom.

Thus, the speed of the flow of polymeric material can be furtherincreased, which allows the working temperature inside the mould (i.e.the temperature which the dose has while being shaped between the maleforming element and the female forming element) to be decreased evenmore significantly.

The flow of polymeric material can be accelerated downstream of thecooling zone.

In particular, the flow of polymeric material can be so accelerated asto have, when exiting a passage conduit arranged downstream of thecooling zone, an advancement speed about 10 times greater than theadvancement speed which the flow of polymeric material had in thecooling zone.

This allows to obtain, near a wall delimiting the passage zone throughwhich the flow of polymeric material passes downstream of the coolingzone, a minimum speed such that any crystals that are formed in thepolymeric material do not adhere to the wall. The flow of polymericmaterial thus exerts a self-cleaning action on the walls with which itcomes in contact downstream of the cooling zone.

In an embodiment, prior the step of severing a dose of polymericmaterial by means of the severing element, there is provided deliveringthe flow of polymeric material by means a nozzle in a position facingthe severing element, so that the severing element can sever the dosefrom the flow. Inside the nozzle, the flow of polymeric material can beaccelerated in such a way as to reach an advancement speed which is 10times greater, and preferably about 30 times greater, than theadvancement speed the polymeric material had in the cooling zone.

This allows to achieve very high advancement speed of the flow ofpolymeric material, thus intensifying the phenomena and effects thathave been described hereinabove.

In an embodiment, there is provided the step of moving the male formingelement and the female forming element relative to one another so as toreach the following configurations in sequence:

-   -   a start-of-forming configuration in which the dose is in contact        with both the male forming element and the female forming        element;    -   a start-of-deceleration configuration in which the male forming        element and the female forming element begin to decelerate        relative to one another, and    -   an end-of-forming configuration in which the dose has been        shaped between the male forming element and the female forming        element until the object is obtained,        wherein, from the start-of-forming configuration to the        start-of-deceleration configuration, the male forming element        and the female forming element move relative to one another with        an average mutual movement speed greater than 10 mm/s.

This ensures that a crystallization induced by the flow (Flow InducedCrystallization), occurs within the mould as well, thereby acceleratingthe crystallization kinetics and further reducing the time required forthe moulded object to reach a semi-crystalline state, in which themoulded object is sufficiently rigid to be removed from the mouldwithout suffering any damages.

In a second aspect of the invention, there is provided an apparatus forproducing an object made of polymeric material, the apparatuscomprising:

-   -   a melting device for melting the polymeric material;    -   a cooling zone for cooling below the melting temperature the        polymeric material molten by the melting device;    -   a severing element for severing a dose of polymeric material        from a flow of polymeric material coming from the cooling zone;    -   a mould for shaping the dose while the latter has a temperature        lower than said melting temperature,        wherein the severing element is positioned downstream of the        cooling zone, so that a path is defined which passes through the        cooling zone and reaches the severing element, and wherein said        path is so dimensioned that the flow of polymeric material moves        with an advancement speed greater than 1.5 mm/s along at least        70% of said path, preferably along at least 90% of said path,        said advancement speed being calculated as an average value in a        cross-section taken perpendicularly to an advancement direction        of said flow.

The apparatus according to the second aspect of the invention allows toobtain the increase in productivity as well as the other advantagespreviously described with reference to the method according to the firstaspect of the invention.

In a third aspect of the invention, there is provided an apparatus forproducing an object made of polymeric material, the apparatuscomprising:

-   -   a melting device for melting the polymeric material;    -   a cooling zone for cooling below the melting temperature the        polymeric material molten by the melting device;    -   a severing element for severing a dose of polymeric material        from a flow of polymeric material coming from the cooling zone;    -   a mould comprising a male forming element and an opposite        element facing the male forming element, the opposite element        and the male forming element being movable relative to one        another for shaping the dose while the latter has a temperature        lower than said melting temperature;    -   a movement device for moving the male forming element and the        opposite element one towards the other, so as to shape the dose        between the male forming element and the opposite element until        the object is obtained, wherein the movement device is so        configured that, between a start-of-forming configuration, in        which the dose starts to be shaped between the male forming        element and a forming surface associated with the opposite        element, and a start-of-deceleration configuration, in which the        male forming element and the opposite element begin to        decelerate relative to one another, the male forming element and        the opposite element have a mutual movement speed greater than        10 mm/s.

Owing to the third aspect of the invention, a very high productivity isachievable.

First of all, a reduction in cycle time occurs since the polymericmaterial is introduced into the mould at a relatively low temperature,which allows to reach more quickly temperatures at which the mouldedobject can be extracted from the mould without being damaged, when themoulded object is cooled.

Furthermore, inside the mould a flow induced crystallization occurswhich allows the kinetics of crystallization to be accelerated, therebyreducing the time needed for removing the object from the mould.

The cooling zone can be defined within a cooling device comprising forexample a static mixer.

The static mixer is a type of heat exchanger which is particularlysimple and effective, and which enables temperature and composition ofthe flow of polymeric material to be homogenized.

In one embodiment, the cooling zone is equipped with a conditioningsystem provided with at least one chamber in which a conditioning fluidcirculates, the conditioning fluid being for example water, steam ordiathermic oil.

This chamber may extend around a passage conduit provided in the coolingzone, particularly within the static mixer, for passage of polymericmaterial.

This allows rapid variations in the temperature of the flow of polymericmaterial which passes through the cooling zone.

In an embodiment, the apparatus comprises an accelerator device,particularly positioned downstream of the cooling zone, for acceleratingthe polymeric material directed towards the severing element.

The accelerator device can be thermally conditioned, particularly withdiathermic oil, or steam, or water.

The apparatus may further comprise a nozzle arranged downstream of thecooling zone for delivering a flow of polymeric material in a positionfacing the severing element, so that the severing element can sever thedoses from such flow.

In an embodiment, the nozzle can be thermally conditioned, particularlywith diathermic oil, or water, or steam.

In this way an optimal control of the temperature of the polymericmaterial can be maintained also downstream of the cooling zone.

In a fourth aspect of the invention, there is provided a method forproducing an object made of polymeric material, said polymeric materialhaving a melting temperature, the method comprising the steps of:

-   -   melting the polymeric material;    -   after the melting step, cooling the polymeric material below the        melting temperature in a cooling zone;    -   severing a dose from a flow of polymeric material coming from        the cooling zone by means of a severing element;    -   obtaining said object by shaping the dose between a male forming        element and a female forming element which move one towards the        other with a mutual movement speed, the dose having a        temperature lower than said melting temperature,        wherein said mutual movement speed is greater than 10 mm/s        between a start-of-forming configuration in which the dose is in        contact with both the male forming element and the female        forming element, and a start-of-deceleration configuration in        which the male forming element and the female forming element        begin to decelerate relative to one another.

This allows to obtain an increase in productivity, as previouslydescribed with reference to the apparatus according to the third aspectof the invention.

It was surprisingly found that, by adopting a mutual movement speed, asdefined above, which is greater than 10 mm/s, defects on the producedobject can be avoided, such as, for example, parts of the object thatare incomplete or a whitening of the object.

Furthermore, the oxygen barrier properties of the finished object areimproved.

In an embodiment, the produced object is made of polyethyleneterephthalate (PET) and is particularly conformed as a preform for acontainer.

If the mutual movement speed is less than 10 mm/s, the preform may haveincomplete parts, particularly in a threaded neck zone of the preformitself. A preform whitening, due to undesired crystallization phenomena,can further occur.

In an embodiment, the above mentioned mutual movement speed with whichthe male forming element and the female forming element move relativelyto each other can be greater than 470 mm/s, particularly if the producedobject is made of polypropylene (PP).

This object can be, particularly but not exclusively, conformed as a cupor a vial.

In an embodiment, the above mentioned mutual movement speed with whichthe male forming element and the female forming element move relativelyto each other can be greater than 900 mm/s, particularly if the producedobject is made of high density polyethylene (HDPE).

This object can be, particularly but not exclusively, conformed as a capfor a container.

The invention will be better understood and carried out with referenceto the appended drawings, which illustrate an embodiment thereof by wayof non-limiting example, wherein:

FIG. 1 is a partial schematic view of an apparatus for producing objectsby compression moulding;

FIG. 2 is a graph showing how the crystallization of a particular typeof polypropylene varies in function of the time;

FIG. 3 is a graph showing how, with reference to the polypropylene ofFIG. 2, the percentage of crystallized mass varies as a function oftime;

FIG. 4 is a graph showing how the time necessary for obtainingcrystallization of 50% of the mass of the material varies depending onthe temperature, with reference to the polypropylene of FIG. 2;

FIG. 5 is a perspective view showing a mixing element which can bepositioned inside a static mixer.

FIG. 1 shows an apparatus 1 for producing an object by compressionmoulding a dose of polymeric material.

The object produced by means of the apparatus 1 may be a cap for acontainer, or a container, or a preform for obtaining a container byblow moulding or stretch-blow moulding, or more generally any concave orflat object.

The polymeric material used by the apparatus 1 can be any polymericmaterial that may be compression moulded, in particular asemi-crystalline material such as polypropylene (PP), high densitypolyethylene (HDPE), polyethylene terephthalate (PET).

Semi-crystalline materials are materials that exhibit, in their solidstate, a fraction of crystalline mass and a fraction of amorphous mass.For semi-crystalline polymeric materials, a melting temperature T_(F)and a crystallization temperature T_(C) may be defined.

In particular, the melting temperature T_(F) is the temperature at whicha polymeric material which is heated, passes from solid state to moltenstate.

The crystallization temperature T_(C) is the temperature at which afraction of material crystallizes during cooling. The crystallizationtemperature T_(C) is lower than the melting temperature T_(F).

To be more precise, the crystallization process does not occur at aspecific temperature, but within a temperature range which is definedbetween a crystallization start temperature T_(IC) and a crystallizationend temperature T_(FC).

Furthermore, the crystallization temperature T_(C), as well as thedifference existing between the crystallization start temperature T_(IC)and the crystallization end temperature T_(FC), are not constant for agiven material, but depend on the conditions according to which thematerial is cooled. In particular, the lower is the temperature at whichthe molten polymeric material is maintained, the faster crystallizationthereof takes place. Moreover, the more quickly the molten polymericmaterial is handled, the more the temperature range at whichcrystallization occurs lowers.

This appears in FIG. 2, which shows the results of an analysis carriedout by differential scanning calorimetry (DSC) on polypropylene samples.The material samples analyzed were brought to a temperature higher thanthe melting temperature, at which temperature they were kept for a fewminutes so as to melt all the crystals present therein. The samples werethen cooled down to a pre-determined temperature and maintained at suchtemperature for the time necessary for each sample to be crystallized.Thus, crystallization times and modes were tested for each sample.

FIG. 2 shows the energy released from the samples analyzed as a functionof time, during the crystallization step.

In particular, the curve indicated by A refers to the sample which wascooled to the lowest temperature, namely 108° C. In this sample, thecrystallization has occurred in a shorter time and within a lowertemperature range than the other samples analyzed. Curve A exhibits anexothermic crystallization peak which is the narrowest one among all thesamples analyzed. This means that the difference between thecrystallization start temperature T_(IC) and the crystallization endtemperature T_(FC) for that sample, is minimum with respect to all othersamples analyzed.

The curve referred to with B is instead relative to the sample which wascooled to the highest temperature, i.e. to a temperature of 115° C. Inthis sample the crystallization process did not occur, because the hightemperature at which the sample was maintained did not allow crystals tobe formed during the period of time in which the sample was observed.

This proves that the polymeric material crystallizes faster if a lowertemperature is maintained.

A similar reasoning applies to the melting process and related meltingtemperature.

FIG. 3, based on data obtained from FIG. 2, shows how the crystallizedmass percentage varies in a sample as a function of time. Each curverefers to a different temperature up to which the sample was cooled,after which the sample temperature was kept constant. In particular, thetemperature of each sample increases by moving from left to right in thegraph. It is noted that, the lower is the temperature at which thesample is cooled, the more is reduced the time required for a 100%crystallization of the sample mass to occur.

A half-crystallization time t_(1/2) can be defined, which is the timeneeded by a sample to have half of the mass thereof crystallized. FIG.4, based on data from FIGS. 2 and 3, shows the half-crystallization timet_(1/2) as a function of temperature at which the sample was maintained.It is noted that, upon increasing of the temperature at which the samplewas maintained, the half-crystallization time t_(1/2) increases.

In summary, the behaviour of a semi-crystalline polymer during meltingand crystallization thereof cannot be univocally determined, but isaffected by the cooling conditions, following which the polymer iscooled. In particular, the lower is the temperature at which the moltenpolymer material is kept, the faster crystallization takes place.

The above considerations derive from studies concerning behavior ofsemi-crystalline polymeric materials which were carried out under staticconditions, that is to say, while the sample analyzed was not undergoingany deformation. Crystallization occurring under these conditions iscalled quiescent crystallization.

Nevertheless, when a semi-crystalline polymeric material is subject todeformation, as it happens when the polymeric material is handled in amachine, for example for being subjected to compression moulding, aphenomenon occurs called Flow Induced Crystallization. While thematerial flows, anisotropic crystallites are formed which are orientedin the flow direction, and this modifies the crystallization kinetics ofthe material with respect to the condition under which the onlyquiescent crystallization occurs.

When a polymeric material is cooled below the melting temperature T_(F)and at same time it is deformed, the quiescent crystallization and theflow induced crystallization combine, thus causing a globally fastercrystallization of the material.

It was noted that, by displacing fast a molten polymeric material, thecrystallization temperature thereof lowers and the temperature rangewithin which crystallization takes place narrows. This is due to thefact that, by keeping the molten polymeric material in an agitatedstate, the polymeric chains of the polymeric material have lesscapability of organizing and solidifying in an ordered configuration.

The phenomena described hereinabove can be used to improve compressionmoulding of a semi-crystalline polymer, particularly in an apparatus 1of the type shown in FIG. 1.

The apparatus 1 comprises a melting device not shown, in particular anextruder device, suitable for melting and extruding the polymericmaterial. The polymeric material is heated inside the extruder device,until the polymeric material reaches a temperature higher than themelting temperature T_(F). Downstream of the extruder device, there isprovided a cooling zone which, in the example shown, is defined inside aheat exchanger 2. The cooling zone is configured for cooling the flow ofpolymeric material coming from the extruder device at a temperaturelower than the melting temperature T_(F).

The heat exchanger 2 may comprise a static mixer. The latter maycomprise a conduit through which the polymeric material passes andwithin which a mixing element 16 of the type shown in FIG. 5 isarranged.

The mixing element 16 comprises a plurality of diverting bars 17arranged in a stationary position for homogenizing the flow of polymericmaterial, both from the thermal viewpoint and, where appropriate, fromthe composition viewpoint. In particular, the diverting bars 17 maydivide the main flow of polymeric material into a plurality of secondaryflows which mix with one another along their path inside the staticmixer.

The heat exchanger 2 is equipped with a conditioning system forcontrolling temperature of the flow of polymeric material downstream ofthe extruder device.

The conditioning system is in particular configured for maintaining thetemperature of the polymeric material lower than the melting temperatureT_(F), but higher than the crystallization temperature T_(C) undernormal operating conditions. Of course, when the apparatus 1 starts towork, the conditioning system associated with the heat exchanger 2 aswell as with the zones downstream of the heat exchanger 2, is controlledin such a way as to heat the polymeric material at a temperature higherthan or equal to the melting temperature T_(F). In this way the residualpolymeric material left inside the apparatus 1, which solidified duringthe period of inactivity of the apparatus 1, can be once again meltedand handled. Subsequently, the conditioning system associated with theheat exchanger 2 as well as with the downstream parts of the latter, iscontrolled in such a manner as to bring temperature of the polymericmaterial to a value between the melting temperature T_(F) and thecrystallization temperature T_(C), which will be kept during normaloperation.

Diathermic oil can be used for the conditioning system as a conditioningmeans.

In particular, the diathermic oil can circulate inside a chambersurrounding the conduit through which the polymeric material flows whenpassing through the static mixer. The chamber can be provided with aninlet 3 through which the diathermic oil can enter into the chamber, andan outlet 4 through which the diathermic oil can exit the chamber. Inparticular, the outlet 4 can be arranged upstream of the inlet 3relative to an advancement direction F, according to which the polymericmaterial advances inside the heat exchanger 2.

In this case, the heat exchanger 2 is therefore a countercurrent heatexchanger.

In order to avoid premature crystallization of the material, the heatexchanger 2 is configured for maintaining the flow of polymeric materialpassing through it at a temperature which is not excessively close tothe crystallization temperature T_(C) despite being sufficiently lowerthan the melting temperature T_(F).

To this end, the diathermic oil passing through the heat exchanger 2 canhave an average temperature T_(O) as indicated below:T _(C)−30° C.≤T _(O) ≤T _(F)−30° C.

However this condition is not essential, and different temperatures ofthe diathermic oil can be satisfactorily used.

The apparatus 1 comprises a severing device 5 for severing meteredquantities or doses of polymeric material from the flow of polymericmaterial coming from the heat exchanger 2.

In particular, the severing device 5 enables the doses to be severedfrom the flow of polymeric material exiting from a nozzle 6 arrangeddownstream of the heat exchanger 2. In the example shown, the nozzle 6is facing upwards. In particular, the nozzle 6 can be so configured thatthe flow of polymeric material exiting from the nozzle 6 is directedalong a vertical direction.

However other arrangements of the nozzle 6 are also possible.

The severing device 5 may comprise a collecting element 7, for exampleconformed as a concave element extending about a vertical axis, thecollecting element 7 being suitable for passing close to the nozzle 6,particularly above the latter, so as to sever a dose of polymericmaterial therefrom. Thus the collecting element 7 acts as a severingelement for severing the dose from the flow of polymeric material comingout of the nozzle 6. The dose remains attached to the collecting element7, which carries the dose towards a mould 8 in which the dose can beshaped in order to obtain the desired object.

The collecting element 7 is rotatable about a rotation axis Y,particularly vertical. To this end, the collecting element 7 can befastened to an end region of an arm 9 which is rotatable about therotation axis Y.

The mould 8 may comprise a female element 10 and a male element which isnot shown. In the example which is being described, the male element isarranged above the female element 10 in the, and aligned with the latteralong a vertical axis. However, further mutual arrangements of thefemale member 10 and the male element are also possible.

FIG. 1 shows only one mould 8. However, the apparatus 1 may comprise aplurality of moulds 8, for example arranged in a peripheral region of acarousel. The carousel can be rotatable about a vertical axis.

The apparatus 1 comprises a movement device for moving the femaleelement 10 and the male element one towards another and alternativelymoving the female element 10 and the male element one away from another.In particular, the movement device is configured for moving the femaleelement 10 and the male element with respect to one another between anopen configuration, in which the female element 10 and the male elementare spaced apart from one another, and an end-of-forming configuration,in which between the female element 10 and the male element a formingchamber is defined, the forming chamber having a shape corresponding tothe object to be obtained.

Between the open configuration and the end-of-forming configuration, thefemale element 10 and the male element can assume further intermediateconfigurations as will be better described below.

The movement device can be associated to the female element 10 only, soas to move the female element 10 with respect to the male element whichis instead maintained in a stationary position. Alternatively, themovement device can be associated to the male element, that is thusmoved with respect to the female element 10, which instead is keptstationary.

Alternatively, the movement device can act simultaneously on both themale element and the female element 10, which are then both moved. Themovement device can be, for example, of the mechanical or hydraulictype. An example of a mechanical movement device is a cam device,whereas an example of a hydraulic movement device is a hydraulicactuator.

In any case, the movement device is configured for moving the maleelement and the female element 10 relative to one another along amoulding direction which in the example illustrated is vertical.

As mentioned above, the collecting element 7 is movable about therotation axis Y. In particular, the collecting element 7 can be in acollecting position, in which the collecting element 7 is arranged abovethe nozzle 6 for removing a dose of polymeric material therefrom. Sincethe dose is in a state of a highly viscous fluid, the dose remainsadherent to the collecting element 7 which, by rotating about therotation axis Y, carries the dose towards the mould 8.

The collecting element 7 can furthermore be in a delivery position inwhich it is arranged above the female element 10 of the mould 8 so as torelease the dose of polymeric material within a cavity of the femaleelement 10, for example with the aid of pneumatic or mechanical means.

It is also possible to adopt collecting elements 7 having a differentconformation from that shown in FIG. 1. By way of example, the severingdevice 5 could comprise a carousel supporting a plurality of collectingor severing elements suitable for interacting with the nozzle 6 insequence, in order to sever respective doses from the nozzle 6.

In an alternative embodiment, which is not shown, the severing elementfor severing the dose may be supported by the mould 8.

It is also possible that the nozzle 6 extends to a rotating distributorwhich carries the molten polymeric material towards a mould, close towhich the dose is severed from the flow of molten polymeric material, bymeans of a severing element.

The apparatus 1 may further comprise a passage conduit 11 interposedbetween the heat exchanger 2 and the nozzle 6. The passage conduit 11may be “L”-shaped.

The passage conduit 11 can be thermally conditioned, so that thetemperature of the polymeric material passing through the passageconduit 11 remains, inside the passage conduit 11, at a controlled valuebelow the melting temperature T_(F).

To this end, the passage conduit 11 may be provided with an outerchamber in which diathermic oil circulates. The outer chamber may beprovided with an inlet opening 12 and an outlet opening 13.

The inlet opening 12 may be arranged downstream of the outlet opening 13relative to the advancement direction F of the polymeric material in theapparatus 1. In this case, the diathermic oil circulates in a directionopposite to the polymeric material.

The temperature T_(O) of the diathermic oil circulating in the passageconduit 11 can meet the condition previously indicated with reference tothe heat exchanger 2, namely:T _(C)−30° C.≤T _(O) ≤T _(F)−30° C.

However this condition shall not be necessarily met. It is also possibleto keep the diathermic oil circulating in the passage conduit 11 at atemperature slightly higher than that of the polymeric material passingthrough the passage conduit 11, in order not to cool the polymericmaterial excessively, especially if the polymeric material was alreadycooled in the heat exchanger 2.

Also the nozzle 6 can be thermally conditioned, in particular by meansof diathermic oil circulating in an interspace arranged near an outerwall of the nozzle 6. This interspace can be provided with an inlet hole14 and an outlet hole 15 respectively for the inlet and outlet of thediathermic oil.

Also in this case the diathermic oil circulating in the interspaceassociated to the nozzle 6 may have an average temperature T_(O) whichmeets the following condition:T _(C)−30° C.≤T _(O≤) T _(F)−30° C.

It may however also happen that the diathermic oil which thermallyconditions the nozzle 6 has a higher temperature, in order not to coolexcessively the polymeric material, particularly if the latter wasalready cooled inside the heat exchanger 2.

More broadly speaking, the diathermic oil which thermally conditions theheat exchanger 2, the diathermic oil which thermally conditions thepassage conduit 11 and the diathermic oil which thermally conditions thenozzle 6, may have temperatures which differ from one another.

Conditioning fluids other than the diathermic oil, e.g. water, steam orother, can also be used.

The apparatus 1 may comprise an accelerator device for accelerating theflow of polymeric material coming from the extruder device. In theexample illustrated, the nozzle 6 acts as an accelerator device, sinceit is provided with passage sections for the polymeric material, whichpassage sections decrease progressively in the advancement direction F,so as to accelerate the flow of polymeric material passing inside thenozzle 6.

Alternatively, or in combination with the above, also the passageconduit 11 can act as an accelerator device, provided that its internalsections are suitably dimensioned for accelerating the flow of polymericmaterial.

In an embodiment which is not shown, it is also possible to provide adifferent accelerator device, placed at any point between the heatexchanger 2 and the nozzle 6.

During operation, the polymeric material is extruded in the extruderdevice, in which the polymeric material is heated to a temperaturehigher than the melting temperature T_(F) thereof.

The molten polymeric material passes from the extruder device into theheat exchanger 2, in which it is cooled to a temperature lower than themelting temperature T_(F), but higher than the crystallizationtemperature T_(C). Subsequently, the molten polymeric material passesthrough the passage conduit 11 and then through the nozzle 6, from whicha flow of polymeric material exits. From this flow of polymericmaterial, a dose is severed by the severing device 5.

In the apparatus 1, a path for the flow of the polymeric material whichwas melted inside the extruder device may thus be defined. This pathpasses through the cooling zone and reaches the severing device 5. Inparticular, the path of the continuous flow of polymeric material startsin the cooling zone and ends as soon as the polymeric material interactswith the severing device 5.

In the example shown, the path mentioned above passes through the heatexchanger 2, inside which the cooling zone is defined, then passesthrough the passage conduit 11 and finally terminates at the outlet ofthe nozzle 6.

The passage cross-sections of the apparatus 1, as well as the pressureand the speed with which the polymeric material exits the extruderdevice, are such that the flow of polymeric material advances from thecooling zone towards the severing device 5 at an advancement speedgreater than 1.5 mm/s.

In particular, the advancement speed of the flow of molten polymericmaterial is greater than 1.5 mm/s along the entire path which, startingfrom the beginning of the cooling zone, reaches the nozzle 6, or atleast along a substantial part of said path, i.e. preferably along atleast 90% of said path, or in any case along at least 70% of said path.

The advancement speed mentioned above is calculated as an average valueof speed in any cross-section taken perpendicularly to the advancementdirection F of the flow of polymeric material, said cross-section beinginterposed between the extruder device and the severing device 5.

Such average value can be calculated by dividing the flow rate ofpolymeric material by the respective passage section, and by determiningthe passage section on the basis of the internal diameter (or moregenerally of the internal dimensions) of the conduit through which thepolymeric material passes For the sake of simplicity, any componentslocated inside the conduit, such as extrusion screws or diverting bars17 were neglected. In other words, for the purposes of calculating thepassage section which will then be used for determining the averagespeed, it is assumed that the conduit through which the polymericmaterial passes is empty.

In the flow of polymeric material flowing along the path which passesthrough the cooling zone and reaches the nozzle 6, two undesiredphenomena can occur, said undesired phenomena depending on the processconditions that are adopted.

A first undesired phenomenon is the solidification of parts of thepolymeric material in contact with the walls of the components of theapparatus 1 in which the above mentioned path is defined, in particularin contact with the walls of the heat exchanger 2. This occurs whentemperature of the conditioning fluid, for example diathermic oil,present inside the heat exchanger 2, is excessively lower thantemperature of the polymeric material flowing through the heat exchanger2. In this case, the polymeric material is excessively cooled inside theheat exchanger 2.

A second undesired phenomenon is bulk crystallization of the polymericmaterial, which occurs if temperature of the conditioning fluid in theheat exchanger 2 is not so low as to cause solidification of thepolymeric material, but is however such as to excessively cool thepolymeric material flowing in the heat exchanger 2.

Both these two phenomena depend on the advancement speed of thepolymeric material, as well as on the temperature of the conditioningfluid, in particular diathermic oil, inside the heat exchanger 2.

In order that the heat exchanger 2 may efficiently remove heat from theflow of polymeric material, the temperature of the polymeric materialdoes not have to be lower than the temperature of the conditioningfluid.

A situation in which the polymeric material is cooled in the heatexchanger 2 until a temperature equal to the temperature of theconditioning fluid is reached, is considered as an optimal workingcondition of the heat exchanger 2. In this situation, the extent ofcooling applied by the heat exchanger 2 to the polymeric material is asgreat as possible.

It was experimentally found that the above mentioned optimal workingcondition of the heat exchanger 2 is reached when the polymeric materialcoming from the extruder device passes through the heat exchanger 2 withan advancement speed, as defined above, of 2.1 mm/s.

This value is optimal in order to obtain a heat exchanger 2 which isshort and compact.

The advancement speed of the flow of polymeric material in the heatexchanger 2 is however greater than 1.5 mm/s, in order to avoid bulkcrystallization phenomena or solidification of the polymeric material.

In the nozzle 6, and possibly also in the passage conduit 11, the flowof polymeric material is accelerated, thus acquiring an average speedgreater than the average speed that it had in the heat exchanger 2.

In particular, in the passage conduit 11, the flow of polymeric materialcan be accelerated in such a way that, at the exit from the passageconduit 11, said flow has a speed 10 times greater than the speed thatthe flow of polymeric material had in heat exchanger 2.

In the nozzle 6 the flow of polymeric material can be accelerated evenmore significantly, so that the flow of polymeric material exits thenozzle 6 with a speed up to 30 times greater than its advancement speedin the heat exchanger 2.

The dose that was severed by the severing device 5 is carried by thecollecting element 7, up to above a female element 10 of a mould 8. Themould 8 is now arranged in the open configuration, in which the femaleelement 10 is spaced apart from the male element.

The collecting element 7 can thus be interposed between the femaleelement 10 and the male element and release the dose into the femaleelement 10.

When the collecting element 7 is located above the female element 10,the dose is detached from the collecting element 7 and deposited intothe cavity of the female element 10. The female element 10 and the maleelement move one towards another owing to the movement device, untilthey reach a start-of-forming configuration in which the dose ofpolymeric material is in contact with both the female element 10 and themale element and starts to be deformed between the female element 10 andthe male element. At this stage, a forming step begins during which theshape of the polymeric material is gradually modified until the shape ofthe desired finished object is obtained.

The female element 10 and male element are moved one towards anotheruntil an end-of-forming configuration is reached, in which the formingchamber defined between the male element and the female element 10 has ashape corresponding to the shape of the object which is to be obtained.At this point, the female element 10 and male element are maintained inthe mutual position that has been reached in order to enable the objectto crystallize and to become enough resistant for being extracted fromthe mould 8 without being damaged.

Shortly before the end-of-forming configuration is reached, astart-of-deceleration configuration can be reached, in which the mutualspeed with which the female element 10 and the male element move towardsone another begins to decrease. This is due to the resistance exhibitedby the polymeric material against being compressed between the femaleelement 10 and the male element.

Between the start-of-forming configuration and the start-of-decelerationconfiguration, the movement device moves the female element 10 and themale element one towards another with a mutual movement speed greaterthan 10 mm/s.

In particular, this speed can be greater than 30 mm/s.

This speed can further be less than 800 mm/s.

During some tests, even advancement speeds of 4000 mm/s weresuccessfully reached. Hence, moulding at least up to this latter valueof mutual movement speed seems successfully possible.

At the end of forming, the mould 8 is opened and the male element andfemale element 10 are moved away from each other. The formed object isthen removed from the mould 8 and a new forming cycle can start.

The dose is inserted into the mould at a temperature T_(LAV) which islower than the melting temperature T_(F) of the polymeric material ofthe dose, but higher than the crystallization start temperature T_(IC)at which crystals would begin forming, in static conditions.

While the polymeric material that constitutes the dose is being shapedbetween the female element 10 and the male element of the mould,temperature thereof is maintained above the crystallization starttemperature T_(IC).

This does not mean that also the temperature of the female element 10and male element of the mould is lower than the crystallization starttemperature T_(IC). The female element 10 and the male element of themould can be provided with respective cooling circuits inside each ofwhich a cooling fluid circulates. Although the temperature of thepolymeric material that is being shaped is lower than thecrystallization start temperature T_(IC), the temperature of the coolingfluid, as well as that of the respective mould elements, can be lower,even significantly, than the crystallization start temperature T_(IC).

When the mould 8 has reached the end-of-forming configuration in whichthe compression moulded object has reached its substantially definitiveshape, the object is cooled below the crystallization temperature T_(C)thereof. The cooling of the object can occur with a cooling rate greaterthan 3.5° C./s, so that solidification takes place as quickly aspossible. When producing bottle caps according to the method and theapparatus described above, it was surprisingly possible to obtain a 50%reduction in cycle time compared to the known methods.

It is believed that this result is attributable to the synergisticcombination of two distinct phenomena.

On the one hand, inserting the dose into the mould 8 at a temperaturelower than the melting temperature T_(F) allows to reduce the timerequired to cool the compression moulded object down to a temperature atwhich the moulded object can be extracted from the mould and handledwithout being significantly deformed.

On the other hand, by subjecting the flow of polymeric material to ahigh speed upstream of the mould 8 and/or inside of the latter, anincrease of the shear rate of the polymeric material, and thus anacceleration of the kinetics of crystallization is attained, because theflow induced crystallization is added to the quiescent crystallizationwhich would occur under static conditions.

In particular, inside the mould 8, the polymeric material flowing in therelatively narrow spaces defined between the female element 10 and themale element, is subjected to a high shear rate, which causes theformation of oriented and aligned crystallization nuclei, on which thecrystals grow.

The two effects described above combine with each other thereby leadingto a result which goes beyond the algebraic sum that each of them wouldhave if taken alone.

The flow induced crystallization is particularly evident for thematerials having a high molecular weight, such as those that are usuallyused for compression moulding. Such materials typically have values ofatomic mass greater than 10.000 Daltons.

The flow induced crystallization is instead less significant formaterials having a low molecular weight, in which a positive effect canstill be obtained by subjecting the material to high shear rates, sincethis allows the material to be cooled more quickly and therefore thecrystallization temperature T_(C) to be decreased.

For the reasons set forth above, it is useful to accelerate thepolymeric material prior to introducing the dose into the mould 8.

The conditioning system associated with the heat exchanger 2 and,possibly, also with the passage conduit 11 and/or the nozzle 6, allowstemperature of the flow of polymeric material coming from the extruderdevice to be accurately controlled, thus helping to prevent an excessivecrystallization outside the mould 8.

Some tests were carried out in order to determine how the mutualmovement speed, with which the female element 10 moves relative to themale element, affects the properties of the formed object.

A first series of tests concerned preforms for container made ofpolyethylene terephthalate (PET). These preforms comprised a hollowbody, having a first end that was closed. At a second end of thepreform, opposite the first end, a threaded neck was provided, thethreaded neck being intended to form a neck of a container.

Initially, preforms having a thickness of the hollow body of 4 mm wereproduced. This thickness corresponds to a maximum value of preformthickness that can currently be found on the market. It wasexperimentally found that, if the female element 10 and the male elementare moved relative to one another (between the start-of-formingconfiguration and the start-of-deceleration configuration) with a mutualmovement speed of less than 10 mm/s, defects tend to be originated onthe preforms. These defects may consist in a preform whitening, as wellas in the generation of preform zones in which no polymeric material ispresent, particularly at the threaded neck. In other words, some partsof the preform were formed incompletely (incompleteness defects), sincethe molten polymeric material was not capable of filling the wholeforming chamber defined between the female element 10 and the maleelement. If, on the other hand, the mutual movement speed, as definedabove, is greater than 10 mm/s, the preforms did not exhibit whiteningor incompleteness defects.

Subsequently, preforms were produced having a thickness of the hollowbody of 2.5 mm, which corresponds to one of the lowest values currentlyavailable on the market.

In this case, the whitening and incompleteness defects did not occurwhen the female element 10 and the male element were moved relative toone another (between the start-of-forming configuration and thestart-of-deceleration configuration) with a mutual movement speed ofgreater than 18 mm/s. Below this value, the preforms were incomplete orexhibited white zones.

A second series of tests concerned production of cups or vials, i.e.containers having a hollow body with a substantially cylindrical orfrustum-conical shape, made of polypropylene (PP).

It was found that, by moving the female element 10 and the male elementrelative to one another (between the start-of-forming configuration andthe start-of-deceleration configuration) with a mutual movement speed ofless than 470 mm/s, the upper part of the object, arranged near acorresponding free edge, was incomplete. In other words, the freeedge—instead of being shaped as a substantially flat circumference—had avalley in which the polymeric material was absent.

If, on the other hand, the mutual movement speed, as defined above, wasgreater than 470 mm/s, the cups produced during the tests did notexhibit incompleteness defects.

A third series of tests concerned production of caps for containers, thecaps being in particular of the kind provided with an inner thread, madeof high density polyethylene (HDPE).

In this case, it was found that, when moving the female element 10 andthe male element relative to each other (between the start-of-formingconfiguration and the start-of-deceleration configuration) with a mutualmovement speed of less than 900 mm/s, a significant percentage ofincomplete caps were produced. In particular, if the mutual movementspeed mentioned above was 460 mm/s, 18% of the produced caps hadincompleteness defects.

If, on the other hand, the mutual movement speed, as defined above, wasgreater than 900 mm/s, all the formed caps were free of incompletenessdefects.

In the example mentioned above, it was in addition found that the capsproduced by using mutual movement speeds greater than 900 mm/s hadoxygen barrier properties increased by 15% if compared with capsproduced by using a mutual movement speed of 460 mm/s.

The experimental results prompt to believe that the improvement inoxygen barrier properties occurs even for objects different from capsand materials different from high density polyethylene.

Furthermore, the increase in the above mentioned mutual movement speedallowed a reduction in the moulding pressure, i.e. in the pressure withwhich the female element 10 and the male element were pushed one againstanother, with consequent decreased stress on the components of theapparatus 1.

The results of the tests referred to above show that, although the valueof mutual movement speed below which defects occur depends on severalfactors, such as geometry and material of the moulded object, theincompleteness and/or whiteness defects are practically certain ifmutual movement speeds of less than 10 mm/s are used.

In the above description, reference was made to a cooling zone definedinside a heat exchanger 2 comprising a static mixer.

This condition is however not essential.

Indeed it may happen that the cooling zone is defined inside a dynamicmixer, i.e. a mixer provided with mixing elements which move duringoperation, rather than inside a static mixer.

Again, the cooling zone can be defined inside a cascade extruder or asatellite extruder, particularly located immediately downstream of theextruder device which melts and extrudes the polymeric material.

The cooling zone could also be defined inside a twin screw extrudersuitably conditioned.

Theoretically, the cooling zone could be defined inside the same meltingdevice which melts the polymeric material, which melting device could beprovided with an end part configured to cool the molten polymericmaterial. In broad terms, the whole length of the apparatus 1 interposedbetween the melting device or extruder device and the severing device 5can be thermally conditioned so as to cool the polymeric material. Insuch a case, the cooling zone begins immediately downstream of the pointwhere the polymeric material is melted and continues up to the nozzle 6.Alternatively, the cooling zone may only affect a portion of theapparatus 1 arranged downstream of the point where the polymericmaterial is melted. In this case, the cooling zone ends upstream of thenozzle 6 and between the cooling zone and the nozzle 6, a maintenancezone is interposed, in which the temperature of the polymeric materialis maintained at desired values.

In this case, the temperature of the polymeric material in themaintenance zone arranged downstream of the cooling zone may becomprised, or substantially comprised, between the crystallizationtemperature T_(C) and the melting temperature T_(F). By the term“substantially comprised” it is meant that at least 90% of the polymericmaterial has a temperature in the range between the crystallizationtemperature T_(C) and the melting temperature T_(F). However smallpolymeric material portions may exist which have a temperature higherthan the melting temperature T_(F), particularly close to the surface ofthe polymeric material flowing in contact with the walls of theapparatus 1.

Additionally, the accelerator device shall not necessarily be present.Even if the accelerator device is present, the flow of polymericmaterial can decelerate before the dose is severed by the severingdevice 5.

In the example shown, reference was made to a situation in which thedose is shaped between a female forming element and a male formingelement belonging to the mould, i.e. the female element 10 and the maleelement which is not shown.

It can further happen that the dose is shaped in contact with an objectwhich is not integrated in the mould, although it behaves like amoulding element while the dose is being formed. This is what happens,for example, in the case of the so called lining, in which the dose isshaped so as to form a liner inside a previously formed cap. Moregenerally, the dose may be moulded inside the cavity of an object, so asto form a component anchored to the object.

In this case, the cap or, more broadly, the object provided with acavity inside which the dose is shaped, acts as a female formingelement, whereas the male forming element is integrated in the mould. Inaddition to the male forming element, the mould also comprises in thisexample a support element facing the male forming element and suitablefor supporting the object inside which the dose has to be shaped duringmoulding.

In broad terms, it can therefore be stated that the mould comprises amale forming element and an opposite element facing the male formingelement. The opposite element may be a female forming element or,alternatively, a support element for supporting an object inside whichthe dose is shaped.

What was previously described with reference to the embodiment in whichthe female forming element is a part of the mould, is to be understoodalso as referring to an embodiment in which the dose is shaped inside ofan object which is not integrated in the apparatus 1 and which acts as afemale forming element.

The invention claimed is:
 1. A method for producing an object made ofpolymeric material, said polymeric material being a semi-crystallinematerial and having a melting temperature, the method comprising thesteps of: melting the polymeric material; after the step of melting,cooling the polymeric material below the melting temperature in acooling zone; using a severing element, severing a dose from acontinuous flow of polymeric material coming from the cooling zone andexiting from a nozzle; obtaining said object by shaping the dose in amould between a male forming element and a female forming element whichmove towards one another with a mutual movement speed, the dose having atemperature lower than said melting temperature, wherein said mutualmovement speed is greater than 10 mm/s from a start-of-formingconfiguration, in which the dose is in contact with both the maleforming element and the female forming element, to astart-of-deceleration configuration, in which the male forming elementand the female forming element begin to decelerate relative to oneanother, and wherein the flow of polymeric material moves with anadvancement speed along a path starting at the cooling zone and reachingthe severing element, said advancement speed being calculated as anaverage value in a cross-section taken perpendicularly to an advancementdirection of said flow, said advancement speed being greater than 1.5mm/s along at least 70% of said path, and wherein the method furthercomprises the step of accelerating the flow of polymeric material priorto severing the dose therefrom, the flow of polymeric material beingaccelerated downstream of the cooling zone so as to have an advancementspeed greater than the advancement speed the polymeric material had inthe cooling zone, whereby an accelerated flow of polymeric materialexits from the nozzle along a direction and the dose is severed from theaccelerated flow and released into the mould while the mould is arrangedin an open configuration.
 2. A method according to claim 1, wherein theflow of polymeric material is so accelerated as to have, at the exit ofa passage conduit arranged downstream of the cooling zone, anadvancement speed 10 times greater than the advancement speed that theflow of polymeric material had in the cooling zone.
 3. A methodaccording to claim 1, wherein the flow of polymeric material isdispensed by the nozzle in a position facing the severing element, sothat the severing element can sever the dose from the flow of polymericmaterial exiting from the nozzle.
 4. A method according to claim 3,wherein, inside the nozzle, the flow of polymeric material is soaccelerated as to have an advancement speed 10 times greater than theadvancement speed the polymeric material had in the cooling zone.
 5. Amethod according to claim 4, wherein, inside the nozzle, the flow ofpolymeric material is so accelerated as to have an advancement speedequal to 30 times the advancement speed the polymeric material had inthe cooling zone.
 6. A method according to claim 1, wherein the step ofcooling the polymeric material comprises using a conditioning fluid forremoving heat from the polymeric material.
 7. A method according toclaim 6, wherein the conditioning fluid is selected from between: water,steam, diathermic oil.
 8. A method according to claim 1, wherein thecooling zone extends from a region in which the polymeric material ismelted, up to the severing element.
 9. A method according to claim 1,and further comprising the step of thermally conditioning the polymericmaterial downstream of the cooling zone and upstream of the severingelement, in order to maintain temperature of the polymeric materialcomprised between the melting temperature and a crystallization starttemperature at which the polymeric material begins crystallizing understatic conditions.
 10. A method according to claim 1, wherein the doseof polymeric material is deformed between the male forming element andthe female forming element while said dose has a temperature higher thana crystallization start temperature at which the polymeric materialbegins crystallizing under static conditions.
 11. A method according toclaim 1, wherein the direction along which the accelerated flow ofpolymeric material exits from the nozzle is a vertical direction.
 12. Amethod according to claim 1, wherein the flow of polymeric material isaccelerated upstream of the nozzle.